This invention relates to photomasks and photomask blanks for use in the production of semiconductors, integrated circuits (IC), large-scale integrated circuits (LSI), and like electronic components.
Photomasks are original plates for printing very minute circuit images or others repeatedly with good precision on wafers of semiconductors, etc. In the prior art, there have been known emulsion masks using inexpensive photographic plates having high resolving power, and also hard masks of excellent durability, including chromium masks, one-side low-reflection chromium masks, both-side low-reflection chromium masks, silicon masks and others.
In general, for high precision purposes, hard masks of excellent resolving power have been employed. Among them, mono-layer chromium masks have high surface reflectance and therefore repetitive reflections are liable to be caused between masks and wafers. For the purpose of avoiding such repetitive reflections, use is practically made of one-side low-reflection chromium masks in which there is provided a reflection preventive layer on one side and further of both-side low-reflection chromium masks, for enabling actuation of auto mask aligners, in which a reflection preventive layer is also provided on the opposite side.
While these multi-layer masks have the advantages of affording a broad exposure latitude in pattern printing on wafers and of enabling actuation of an auto aligner such as that of Casper Co.'s type, they have a drawback in that it is difficult to form images of good quality and durability on these masks, themselves, as compared with mono-layer type chromium masks.
More specifically, this problem is due to the difference in etching speed between the chromium layer and the chromium oxide layer laminated on the upper or lower surface of the chromium layer when the masking layer in the multi-layer structure is to be patterned by etching according to the well-known photolithography process. Usually, the etching speed in the chromium oxide layer is slower by several times than in the chromium layer. Therefore, since the etching speed is faster in the lower chromium layer than in the upper chromium oxide layer, the undercut quantity of the chromium layer is greater, thereby forming a visor of the chromium oxide layer at peripheral portions of images.
On the other hand, because the etching speed varies greatly at an intermediary portion of the entire film, the film as a whole is prone to be unevenly etched, whereby the sharpness of image is impaired, and formation of edges like burrs or mouse nip is frequently caused. Formation of visors around patterns is a great problem because such visors protrude out of the pattern edges as films of some hundred angstroms in thickness, which are very brittle and readily fractured. Practically, fracture will occur in various washing procedures during fabrication or usage of masks or in contact with or peel-off from the resist surface during transfer of images, thereby causing edges like discontinuous burrs or mouse nip to be formed around the images.
The above phenomenon is in contradiction to use for an acceptable high precision transfer and is particularly incompatible with the purpose of producing high performance semiconductor devices such as V-LSI. On the other hand, formation of visors around mask patterns also means that the optical concentration gradient of the image edges is microscopically gentle, whereby dimensional accuracy at the time of transfer is readily affected by exposure conditions to cause lowering of precision.
As a method generally practiced for overcoming these problems, chromium films and chromium oxide films are formed according to the methods which are different from each other. For example, a chromium film of the lower layer is formed by sputtering, while a chromium oxide film of the upper layer is deposited by vacuum evaporation. This is based on the principle that the chromium oxide layer obtained by vacuum evaporation can be etched faster than that formed by sputtering, its etching speed being nearer to the etching speed of the chromium film formed by sputtering. The reason why there is such a difference in etching speed between the methods has not yet been clarified, but it is presumably because sputtering can produce films which are generally more dense as compared with those formed by vacuum evaporation, which are sparse and lower in degree of oxidation.
If, instead of resorting to this method, the chromium and chromium oxide layers are laminated by only the ordinary sputtering method or vacuum evaporation method, visors or burrs will be formed since the etching speed of the chromium oxide layer is relatively higher than that of the chromium layer as stated above. A critical drawback resulting from a combination of the sputtering method and the vacuum evaporation method is a remarkably low productivity due to the steps of first forming a film by sputtering, thereafter taking the product once out of the vacuum, and again performing vapor deposition of a film in a vacuum evaporator. Further, with respect to quality, also, a chromium oxide film is formed again on the chromium film which has been once exposed to the atmosphere, whereby the chromium oxide film obtained has poor reproducibility of the film properties. Another disadvantage is that there is a great probability of dust being drawn in to adhere on the chromium film when returning from vacuum to atmospheric pressure in the apparatus after formation of the chromium film of the lower layer, whereby pinholes are liable to be formed in the chromium oxide film of the upper layer. If an intermediate washing step is introduced for avoiding this probability, the number of steps will be increased, and the quality of the product may be unstable, varying with such washing operations.
Of course, it is possible to vary to some extent the etching speed of a film by changing the conditions for the fabrication of the film according to each of the sputtering and vacuum evaporation methods thereby to change to some extent the density or degree of oxidation of the film, but the range of such a variation is very narrow, and a drastic change in conditions may result in impairment of the basic properties of photomasks such as film strength, chemical resistance, and othrs, or in marked lowering of productivity. For example, when a chromium oxide film is formed by sputtering, it is possible to form a chromium oxide film with lower degree of oxidation by controlling the partial pressure in the atmosphere, the film having an etching speed similar to that of a chromium film. However, it is extremely difficult to control the sputtering conditions for formation of this film in an intermediary oxidized state, and the reproducibility is poor. Further, if the degree of oxidation is too low, the refractory index of the film is increased to impair the effect of prevention of reflection as originally intended. The difficulty in controlling the formation of the above intermediarily oxidized film is further pronounced in the case of reactive vapor deposition.